1) Field of the Invention
The present invention relates generally to a method of recycling spent nuclear fuel (SNF). More particularly, the present invention relates to a method of recycling SNF via an atmospheric-pressure volume plasma fluoride volatility process that removes uranium in the form of gaseous uranium hexafluoride (UF6) while retaining the plutonium in the form of solid plutonium tetrafluoride (PuF4). The remaining products including PuF4, which are reduced by volume significantly, will be stored or recycled by conventional other technologies such as PUREX.
2) Description of Related Art
Light-water nuclear reactors in the U.S. generate about 2000 metric tons of spent nuclear fuel (SNF) every year. SNF is nearly 96 percent uranium. Separating the uranium from the SNF would dramatically reduce the volume of SNF that would subsequently need to be disposed of. Aqueous chemical recovery processes such as PUREX dissolve the SNF using liquid acids, generating large amounts of liquid radioactive waste as a result.
Fluoride volatility methods have been used for uranium separation and recovery. It is known in the art to produce fluorides of uranium and plutonium using a “flame fluoride volatility” process. In such a process, metals are reacted with fluorine gas in a high temperature reactor to form the fluorides. Disadvantages of the flame fluoride volatility process are material problems associated with the high temperature corrosive environment and the production of PuF6, (a gas formed at high temperatures) rather than the solid PuF4 (formed at lower temperatures).
U.S. Pat. No. 4,202,861 discloses flame fluorination followed by fluidized bed fluorination of irradiated nuclear fuels to produce uranium hexafluoride (UF6) and plutonium hexafluoride (PuF6). The method teaches re-fluorination of any plutonium tetrafluoride (PuF4) to produce plutonium hexafluoride (PuF6) at high temperature (>2000 deg K).
U.S. Pat. No. 4,522,794 discloses flame fluorination of UO2 and PuO2 to form hexafluorides. PuF4 is converted to PuF6 through the use of catalysts.
U.S. Pat. No. 6,699,398 discloses plasma fluorination of actinide oxides from a substrate surface. Use of rf and microwave plasma generators at low pressure (sub-atmospheric) were disclosed.
U.S. Pat. No. 7,172,741 discloses flame fluorination of spent nuclear fuel to produce UF6. Any PuF4 that is produced is recovered via two methods, and reconverted to PuF6. The first method of recovering PuF4 is adsorption onto an adsorption tower, followed by oxidative conversion, and re-fluorination (to form PuF6). In the second method, the PuF4 is captured at the bottom of the reactor tower, where it is oxidated, and re-fluorinated (to form PuF6).
U.S. Pat. No. 7,208,129 discloses a three step flame fluorination of spent oxide fuel: i) HF fluorination to form UF4 and PuF4; ii) low temperature F2 fluorination to convert UF4 to UF6; iii) high temperature F2 fluorination to form UF6 and PuF6.
U.S. Pat. No. 7,323,153 discloses a two step flame fluorination process: i) HF fluorination to form UF4 and PuF3; ii) F2 fluorination to form UF6 and PuF6.
U.S. Pat. No. 7,445,760 discloses fluorination of SNF to remove a portion of the uranium as UF6. The remaining products (uranium, plutonium, etc.) are dissolved in nitric acid to recover the plutonium.
Published U.S. patent application 2010/0126874 discloses flame fluorination of spent nuclear fuel to form UF6, enabling removal of the bulk of the uranium. The remainder is processed by an electrolysis method to separate and extract remaining metals.
Journal article, “Etching of UO2 in NF3 RF plasma glow discharge”, Veilleux et al, Journal of Nuclear Materials, vol. 277, (2000), pages 315-324, discloses the use of a low pressure (sub-atmospheric) RF plasma (NF3 gas) to decontaminate depleted uranium oxide from stainless-steel substrates.
Journal article, “Decontamination Process Using CF4—O2 Microwave Discharge Plasma at Atmospheric Pressure”, Windarto et al, Journal of Nuclear Science and Technology, Vol. 37, No. 9, pages 787-792, (2000), discloses use of atmospheric pressure microwave discharge plasma to fluorinate cobalt oxide deposited on a surface.
Journal article, “Etching of uranium oxide with a non-thermal atmospheric pressure plasma”, Yang et al, Journal of Nuclear Materials, Vol. 324, pages 134-139, (2004), discloses use of atmospheric RF discharge plasma to etch uranium oxide deposited on a surface.